• Topics: Dobbs, Cynnamon (student) | Hagan, Donald (student) | Andreu, Michael G | Escobedo, Francisco J | Toor, Gurpal S | Soil and Water Science | Hillsborough County Soil and Water | Urban Soils

Key Physical and Chemical Properties of Tampa's Urban Soils¹

Introduction

Scientists and land managers are increasingly aware of the ecosystem services provided by urban forests and soils. Nonetheless, urban soils have received much less research attention than have agricultural and forest soils, and remain poorly understood. Agriculture and forest soils are frequently studied, but urban soil is generally not. In many cases, urban soils are simply assumed to be homogenous, highly modified from natural conditions, or of low fertility. Further, the physical and chemical properties of urban soils are poorly understood. Most soil survey maps do not describe urban soils; usually they are shown as blank areas in the landscape. Recent studies in Florida, however, reveal that urban soils are highly variable, ranging from highly modified to relatively undisturbed (Dobbs, 2009; Hagan et al., 2010). Despite this variability, a few trends and patterns in soil characteristics are evident in urbanized landscapes across the United States (see Pouyat et al., 2007 for characteristics of urban soils in Baltimore, Maryland). For sustainable management of urban areas and to better assess their ecosystem services, we must learn why and how soil properties vary across an urban and urbanizing landscape.

Recent studies conducted in Tampa have shown how soils are affected by urbanization (Dobbs, 2009). Tampa is the second-most populous urban area in Florida with 2.7 million residents. Tampa has numerous older developed urban areas as well as other areas in various states of urban and suburban development. This publication will map and provide an overview of four key soil properties: bulk density, organic matter, phosphorus, and lead. It will explain how these properties vary across different land uses in Tampa using geostatistics (a type of statistics that uses spatial data) and geographical information systems. The soil properties described in this publication are important for plant growth and stormwater runoff generation, which can be used for planning purposes in the region. The researchers collected and analyzed the first ten centimeters of the surface soil from 106 locations across Tampa. Site-specific information is limited due to the relatively small sample size, but the maps will provide a general overview of urban soils in Tampa and the watershed-level information can be valuable to determine variability in these soil properties and to supplement information from soil surveys. This information should be of use to land-use planners, water management districts, Extension agents and municipalities

Physical and Chemical Properties of Urban Soils

Bulk Density

Generally, bulk density values near 1.3 g/cm³ are considered optimal for plant growth and water infiltration. Soil bulk density is a measure of soil compaction—the mass of dry soil (g) per unit volume (cm³). Knowing the bulk density of soil can help estimate plant growth, water infiltration, stormwater runoff volume, and erodibility. For example, if soil bulk density increases or the soil is compacted (meaning more soil in less volume), then less water will infiltrate and there will be more stormwater runoff. (Even when bulk density is lower, a surface crust may form on the soil when it is saturated and then dries out, and this crust can also result in low infiltration and high runoff.) In a compacted soil, plant roots will not penetrate with depth. Plants in compacted soil usually grow poorly because they are unable to mine water and nutrients from subsoil. Traffic—by pedestrians and cars as well as heavy building and roadway construction equipment—can disturb or compact the soil and increase soil bulk density, so bulk density is usually greater in heavily urbanized areas compared with natural areas like forests and wetlands. Vegetation cover is not necessarily a good predictor of bulk density. For example, urban soils covered by turfgrass are among the most compacted (Hagan et al., 2010) because before planting turfgrass, areas around buildings are compacted to stabilize the site and to reduce subsidence. Later on, fill material (usually sand) is applied on top of the compacted sites to grow turfgrass.

Figure 1. 

Patterns of soil bulk density (g/cm³) in Tampa.

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Figure 2. 

Measured soil bulk density (g/cm³) by land use in Tampa.

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The average bulk density of Tampa's urban soils was 1.02 g/cm³, which suggests that most soils in Tampa are not compacted. Figure 1a shows that bulk density values in Tampa were highly variable and ranged from 0.59 g/cm³ (very low) to 1.33 g/cm³ (about average). In general, bulk density was highest in the soil samples collected from institutional, residential, vacant and wetland areas, where bulk density values greater than 1.1 g/cm³ were common (Figure 1b). Soils in highly developed residential areas southwest of downtown Tampa were more compacted than surrounding areas, exceeding 1.0 g/cm³ across a large area. The least compacted soils (1 g/cm³ or less) were in industrial, suburban, and agricultural areas.

Organic matter

Soil organic matter is the decomposed and partially decomposed remnants of living organisms (primarily plants) found in soils. Organic matter in soils is important for many reasons. First, because organic matter stores several essential plant nutrients like nitrogen and phosphorus, it promotes plant growth. Furthermore, because it is light and porous, organic matter increases soil water retention capacity, promotes formation of soil structure, and decreases bulk density. Soil organic matter also increases the ability of soil to tightly retain and degrade contaminants, thereby reducing the likelihood of losses and offsite water contamination. The accumulation of soil organic matter, which is approximately 50% carbon, can help mitigate climate change by storing atmospheric carbon dioxide. Since organic matter is primarily of plant origin, it tends to accumulate in urban or forest areas with tree or grass cover, or in poorly drained areas like wetlands, where organic materials decompose relatively slowly. More information about importance of soil organic matter and how to build organic matter content in soils can be found in http://edis.ifas.ufl.edu/mg454.

Soil organic matter content in Tampa averaged 4.8% of total soil weight. Like other soil properties, organic matter content was highly variable, with a range from 0.9% to 13.2% (Figure 2a). The lowest organic matter values were found in suburban areas in north and northeastern Tampa. Intermediate values for soil organic matter were generally found outside of the highly developed urban core. In general, the highest organic matter contents were found in residential and agriculture areas and lowest in vacant and utility land uses (Figure 2b). The elevated organic matter contents reported in industrial areas could be attributed to past land uses, or to random samples that were located on heavily vegetated sites within these areas.

Figure 3. 

Patterns of soil organic matter contents (%) in Tampa.

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Figure 4. 

Measured soil organic matter contents (%) by land use in Tampa.

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Phosphorus

Phosphorus is an essential plant nutrient found naturally, in varying amounts, in most soils. In general, "young" urban soils—especially those consisting of sandy fill material—are often depleted of phosphorus, while soils in older urban areas are more likely to be high in phosphorus. Urban soils that have received inputs of phosphorus from fertilizer and organic amendments including composts generally have high phosphorus content. From an urban soils management perspective, soil phosphorus is an important consideration because phosphorus runoff and leaching are primary causes of surface and groundwater pollution in Florida. Many soils in Tampa have high natural phosphorus levels and many areas on the outskirts of Tampa have active commercial phosphate mining operations for phosphorus fertilizer manufacture. Even though most Florida soils are coarse textured (sandy) with limited ability to retain phosphorus, phosphorus fertilizer should only be applied if soil test indicates that it is needed.

Soil phosphorus concentrations across Tampa were measured with a soil test calibrated with plant availability known as Mehlich-1. Phosphorous concentrations proved to be highly variable, averaging 90 mg/kg (or ppm) and ranging from 1 to 1095 mg/kg (Figure 3a). The highest values of soil phosphorus (more than 200 mg/kg) were found in highly urbanized commercial and residential areas near downtown Tampa (Figure 3a). These high values could be natural (Tampa has naturally phosphorus-rich soils) or anthropogenic (increased by fertilizer or waste inputs). According to the Florida Phosphorus Index, all soils in Tampa classified by the USDA as "urban" have very high potential for phosphorus runoff, while the phosphorus runoff potential for lesser developed areas ranges from low to very high. Several factors that can affect runoff and leaching potential in soils include: (1) the presence or absence of impervious surfaces (roads, parking lots etc.) and near-surface phosphorus-rich parent materials that can reduce infiltration, (2) loamy or clayey layers that can bind phosphorus and inhibit leaching, and (3) absence of vegetation that can increase phosphorus loss due to soil erosion. Chahal et al. (2010) reported that the average phosphorus content in urban residential soils of Pinellas County (including parts of west Tampa) was 132 mg/kg, with a range from 11 to 471 mg/kg. They suggested that since urban residential soils have significant grass cover, the likelihood of phosphorus runoff might be substantially lower than soils without grass cover despite their higher phosphorus content.

Figure 5. 

Patterns of soil phosphorus contents (mg/kg) in Tampa.

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Figure 6. 

Measured soil phosphorus (Mehlich 1-extractable) contents (mg/kg) by land use in Tampa. M-1 extractable values >30 mg/kg receive no phosphorus application recommendations.

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Lead

Lead is a heavy metal that occurs naturally, usually in low concentrations, in many soils. In urban areas, however, elevated lead is common and is primarily attributed to industrial emissions and past use of lead in gasoline, paints, and pipes (Chirenje et al., 2004). Lead is not very mobile in soil, so it tends to accumulate with time rather than leach from soil. Lead contamination is a major cause for concern because it has been linked to human behavioral problems, learning disabilities, seizures, and death. Young children are particularly at risk because they are most likely to come in contact with or ingest lead-contaminated soils. The United States Environmental Protection Agency has established lead clean-up target levels of 400 and 920 mg/kg for residential and commercial areas, respectively (http://www.epa.gov/lead).

Soil lead concentrations in Tampa averaged 56 mg/kg, and ranged from 12 to 943 mg/kg. The highest values, which approach or exceed EPA clean-up target levels, were found in localized areas near downtown Tampa, which is an older and highly developed commercial, industrial and residential zone (Figure 4a, 4b). Vacant land in this area has by far the highest soil lead content. The lower contents found elsewhere in the city are of minimal concern, and it is unlikely that additional lead will accumulate in these areas.

Figure 7. 

Patterns of soil lead contents (mg/kg) in Tampa.

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Figure 8. 

Measured soil lead contents (mg/kg) by land use in Tampa. The US Environmental Protection Agency's established lead clean-up target level is 400 mg/kg for residential and 920 mg/kg for commercial areas (http://www.epa.gov/lead).

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Conclusions

In general, urban soils in Tampa have low bulk density, adequate organic matter, and low lead contents. Phosphorus concentrations, however, are elevated in many areas, particularly in institutional and residential land uses. Our maps indicate that these properties are highly variable across the urban landscape, which suggests that Tampa's urban soils are diverse, complex, and affected by numerous human and environmental factors. Improving our understanding of these factors is an essential first step in interpreting soil surveys, improving urban land use planning and decision making, mitigating the effects of climate change, and sustainably managing the urban soil resource.

References

Chirenje, T., L. Q. Ma, M. Reeves, and M. Szulczewski. 2004. Lead distribution in near-surface soils of two Florida cities: Gainesville and Miami. Geoderma. 119:113–120.

Chahal, M. K., G. S. Toor, and P. Brown. 2010. Trace metals and polycyclic aromatic hydrocarbons in an urbanized area of Florida. Soil and Sediment Contamination. 19:419–435.

Dobbs, C. 2009. An Index of Gainesville's Urban Forest Ecosystem Services and Goods. University of Florida MS Thesis.

Hagan, D. L., C. Dobbs, and F. Escobedo. 2010. Florida's urban soils: Underfoot yet overlooked. Florida Cooperative Extension Service, University of Florida, Gainesville, FL.

Pouyat, R.V., I. D. Yesilonis, J. Russell-Anelli, and N. K. Neerchal. 2007. Soil chemical and physical properties that differentiate urban land-use and cover types. Soil Science Society of America Journal. 71, 1010–1019.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, Dean.

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